This invention relates to a light wave guide which has a tapered coupling section and which enables highly efficient coupling between a light wave guide and a laser or a fiber used in the field of optical information processing and in the field of applied optical measurement control applied with a coherent light source, and to a wavelength converting element using this wave guide.
TE/TM mode splitters and wavelength converting elements have conventionally been manufactured by forming a light wave guide on LiNbO.sub.3 based on a proton exchange method. In wave guides thereby manufactured, the difference between the refractive indices of the wave guide and the base is large (.DELTA.ne&gt;0.1), and the thickness is very small, 0.4 to 0.6 .mu.m if the wave guide is arranged to propagate light in a single mode. To improve the efficiency of coupling with the wave guide, therefore, tapering a section through which light is introduced into the wave guide has been proposed.
A conventional light wave guide is known in which a section through which incident light is introduced into a wave guide section is widen by tapering or the like (This type of light wave guide is hereinafter referred to simply as "tapered wave guide"). FIG. 10 shows the basic construction of this conventional tapered wave guide. This wave guide includes a ferroelectric material substrate 21, a wave guide 22, a tapered wave guide 23, and an input section 24.
In the conventional tapered wave guide having this construction, the shape and the area of the section through which light is introduced are changed so that the distribution of the wave front of exciting coherent light in the wave guide matches with the electric field of the guided mode of propagation through the wave guide, thereby obtaining a high efficiency of coupling with the wave guide with respect to coherent light.
A method of manufacturing such a tapered wave guide is disclosed by J. C. Campbell on pages 900 to 902 of Applied Optics, March 1979, volume 18. In this method, as shown in FIG. 11, a tapered incident light wave guide is formed by gradually immersing substrate 21 in a solution 25 of AgNO.sub.3 so that the diffusion depth is changed. In the case of forming a wave guide on LiNbO.sub.3 substrate 21, benzoic acid is used as solution 25 and heating is effected at about 200.degree. C. to form a tapered wave guide. In FIG. 11, a heater and a beaker are indicated by 26 and 27, respectively.
In the above-described arrangement, however, it is difficult to sufficiently widen the input section 24 of the tapered incident light wave guide relative to the non-tapered wave guide. While the wave guide is widened in directions of width and depth, the section through which light propagates is abruptly reduced at a position where the wave guide is narrowed by tapering. Therefore the power density in the wave guide is abruptly increased at this position so that optical damage is caused, if the light is converged at this position. In the tapered section, both the width and the depth are increased so that a multi-mode wave guide is formed, but coupling occurs between guided multiple modes by a change in refractive index caused by optical damage, resulting in occurrence of a coupling loss at the position where the single-mode wave guide and the tapered section are connected.
The above-described method of manufacturing a tapered wave guide entails a problem in that the temperature of a portion not immersed in the solution is lowered by vapor during the high-temperature heat treatment, and in that the section through which light is introduced cannot be formed in accordance with the design with desired reproducibility.